1. Field of the Invention
The invention relates to a secondary battery state-of-charge estimating apparatus, a secondary battery state-of-charge estimating method, and a recording medium readable by a computer which stores a program for directing the computer to execute a routine according to the secondary battery state-of-charge estimating method. More specifically, the invention relates to an apparatus for estimating the state-of-charge of a secondary battery mounted in an electric vehicle or a hybrid vehicle, a method for estimating a state-of-charge of a secondary battery, and a recording medium readable by a computer which stores a program for directing the computer to execute a routine according to the method for estimating the state-of-charge of the secondary battery.
2. Description of the Related Art
Electric vehicles (hereinafter also abbreviated to “EVs”) and hybrid vehicles (hereinafter also abbreviated to “HVs”) have recently gained attention as environmentally friendly vehicles. EVs and HVs run by driving a motor using a secondary battery which is mounted in the vehicle as the energy source.
A nickel metal hydride battery or lithium battery or the like, which have superior fundamental characteristics such as energy density, output characteristics and cycle life characteristics is generally used as the secondary battery in an EV or HV. When this type of secondary battery is used as the energy source for a motor for running a vehicle, an accurate estimate of the state-of-charge (hereinafter also abbreviated to “SOC”) of the secondary battery is crucial to calculate the possible running distance with the secondary battery and to prevent over-discharge of the secondary battery.
FIG. 27 shows the relationship between the SOC and open circuit voltage (hereinafter also abbreviated to “OCV”) of the secondary battery.
Referring to the drawing, because the correlation between the SOC and the OCV is fixed, it is possible to calculate the SOC from the OCV using this relationship. That is, it is possible to detect the battery voltage using a voltage sensor and then calculate the SOC based on the OCV calculated from the detected battery voltage.
In this case, the OCV here is the voltage when the charge and discharge current of the secondary battery is 0 (amperes), i.e., it is the voltage between open terminals excluding the polarization effect inside the battery. That is, the OCV does not necessarily match the value of the battery voltage detected by the voltage sensor due to the polarization effect inside the battery. The relationship between the battery voltage V detected by the voltage sensor and the OCV can generally be expressed with Expression 1 below.V=OCV+VR+VDYN   (1)where VR represents to a voltage drop due to internal resistance in the battery, and VDYN represents polarization voltage.
The voltage drop VR depends on the charge and discharge current and is 0 when the terminals are open. On the other hand, the polarization voltage VDYN depends on such factors as the charge and discharge state, amount of current, and temperature at that time. Also, when the secondary battery is left with the terminals open, the polarization voltage VDYN value decreases over time until it finally becomes 0 after enough time has passed. The battery voltage V when the terminals are open and the polarization voltage VDYN is 0 matches the OCV.
As described above, in order to obtain the SOC it is necessary to obtain the OCV, and in order to obtain the OCV it is necessary to accurately estimate the polarization voltage VDYN in particular. As shown in FIG. 27, the amount of change in the SOC is large with respect to the change in the OCV around the working voltage of the secondary battery (i.e., around 15V with a 12-cell battery). Accordingly, accurately estimating the polarization voltage VDYN contributes greatly to improving the accuracy in estimating the SOC. FIG. 28 is a view illustrating the shift in the polarization voltage. Referring to the drawing, the vertical axis denotes the polarization voltage and the horizontal axis denotes time. The temperature is constant. The periods of time T1 to T2 and time T3 to T4 are periods during which the EV or HV is running and the secondary battery is charging and discharging. The periods of time T2 to T3 and time T4 onward are periods during which the EV or HV is not being used and the secondary battery is disconnected from the load (hereinafter, each these periods will also be referred to as a “not-in-use period”).
As the secondary battery charges, the polarization voltage increases in the positive direction (hereinafter, when the polarization voltage is positive, it will also be referred to as “charge polarization”). On the other hand, as the secondary battery discharges, the polarization voltage increases in the negative direction (hereinafter, when the polarization voltage is negative, it will also be referred to as “discharge polarization”). During the not-in-use period, the polarization voltage generated at that time does not immediately disappear, but rather gradually decreases toward 0.
As described above, because the polarization voltage depends on the charge and discharge current and temperature, as well as on the charge and discharge history, if the vehicle is running, it is possible to calculate the polarization voltage with a control apparatus using a polarization voltage model derived in advance. During the not-in-use period, however, it is not possible to calculate the change in the polarization voltage during that time due to the fact that the power of the vehicle is off.
In view of this problem, an SOC estimating apparatus disclosed in JP(A) 2001-272444 is provided with a timer that times the not-in-use period. When the not-in-use period that was timed is within a predetermined set period, the OCV or SOC is corrected according to the length of the not-in-use period. When the not-in-use period that was timed is longer than the predetermined set period, it is determined that the polarization has disappeared.
While the apparatus disclosed in JP(A) 2001-272444 does enable the SOC after the not-in-use period to be accurately calculated because the polarization voltage is corrected according to the length of the not-in-use period, it also requires that a timer be provided to time the not-in-use period. Providing a separate timer in this way increases the cost of the control apparatus.
Further, when a timer is not provided and the polarization voltage before the not-in-use period has started is stored and that stored polarization voltage is used after the not-in-use period has ended, the estimation accuracy of the SOC does not deteriorate much when the not-in-use period is short because the change in the polarization voltage is small. When the not-in-use period is long, however, the polarization voltage disappears so a large error is generated in the estimated value of the SOC when the stored value of the polarization voltage was large.
Also, if a timer is not provided and the polarization voltage after the not-in-use period is always made 0, the polarization voltage actually disappears when the not-in-use period is long so there is no problem. When the not-in-use period is short, however, the polarization voltage remains so the estimation accuracy of the SOC greatly deteriorates when the polarization voltage before the not-in-use period was large.